The present invention relates to a surface-acoustic-wave filter of the resonator type, useful as a radio-frequency filter in a portable communication device.
Compact, lightweight portable telephone sets and other portable communication equipment have been undergoing intensive development in recent years. Small, lightweight, high-performance components are required.
Surface-acoustic-wave (SAW) filters are small and lightweight, require no adjustment, and can be manufactured in large quantities by the same photolithographic technology that is used to manufacture semiconductor devices. Moreover, the amplitude characteristics and phase characteristics of a SAW filter can be controlled independently. SAW filters are finding increasing use as picture-signal intermediate-frequency (PIF) filters, vestigial sideband (VSB) filters, and other types of communication filters, and as filters for digital signal processing.
SAW filters are also used in the radio-frequency (RF) sections of portable telephone sets, where they have played a significant role in the achievement of improved performance and small size. The type of SAW filter that has been mainly used is a ladder filter employing SAW resonators. In addition to small size, this type of filter offers the advantages of narrow bandwidth, low insertion loss in the passband, and high attenuation in the adjacent stopbands, and it does not require a matching circuit.
FIG. 1 shows a conventional ladder filter of the SAW resonator type, having two series-arm SAW resonators 11, 12, and one shunt-arm or parallel-arm SAW resonator 13. The series-arm SAW resonators 11, 12, referred to below as SR1 and SR2, are coupled in series between an input terminal 14 and an output terminal 15. The input terminal 14 is paired with a ground or earth (E) terminal 16 to form the input port of the filter. The output terminal 15 is paired with another ground terminal 17 to form the output port. The shunt-arm SAW resonator 13, referred to below as PR1, is coupled on one side to both ground terminals 16, 17, and on the other side to a node located between the two series-arm SAW resonators SR1 and SR2.
Each SAW resonator SR1, SR2, PR1 comprises an interdigital transducer (IDT, not visible) flanked by a pair of grating reflectors (not visible). FIG. 2 shows an equivalent circuit of one half of the SAW filter in FIG. 1, representing each SAW resonator as a capacitance C0, equivalent to the capacitance of the IDT, paralleled by an inductance L in series with a capacitance C. This is also the equivalent circuit configuration that is used to represent a crystal resonator. A resonator that can be represented in this way has a reactance characteristic with both a resonance frequency and an antiresonance frequency. A bandpass filter can be created by a ladder configuration of such resonators, in the same way that an inductor-capacitor (LC) bandpass filter is configured, by making the antiresonance frequency of the shunt-arm resonator PRE11 substantially equal to the resonance frequency of the series-arm resonator SRE10. SRE10 represents SR1 in FIG. 1, while PRE11 represents half of PR1 in FIG. 1.
The configuration in FIG. 1 is known as a T-type filter configuration. A T-type filter has relatively small attenuation, but the attenuation can be increased by adding more SAW resonator stages.
Portable telephone systems and other mobile communication systems are becoming increasingly diversified. Increasing numbers of channels, diminishing gaps between transmitting and receiving frequency bands, and widening passbands are the order of the day. Because of the advantage of their small size, SAW filters that can meet the necessary requirements are highly desirable, but a SAW filter of the conventional ladder type can satisfy only a limited range of requirements.
For example, portable telephones in a widely promoted code division multiple access (CDMA) system transmit in one band (824 to 849 megahertz or MHz) and receive in another band (869 to 894 MHz). The RF filters of these portable telephone sets must meet insertion-loss and attenuation requirements in these bands, and must also meet requirements for attenuation of spurious signals at two and three times the center frequency (2fc and 3fc) of these bands. SAW filters of the conventional ladder type cannot meet these spurious-signal attenuation requirements.
A conventional solution to this problem is to provide a separate lumped-constant filter or a quarter-wave (xcex/4) stripline inside the RF package to reject the spurious signal components, but it would obviously be preferable to have the spurious components eliminated by the SAW filter itself.
It is accordingly an object of the present invention to provide a SAW filter that meets requirements for spurious-signal attenuation at multiples of the center frequency.
The invented SAW filter has a T-type configuration with a series arm, a shunt arm, an input terminal, an output terminal, and a ground terminal. The series arm has a first SAW resonator and a second SAW resonator coupled in series between the input terminal and the output terminal. The shunt arm has a third SAW resonator coupled between the ground terminal and a node located between the first and second SAW resonators on the series arm. The SAW filter also has a fourth SAW resonator coupled between the input terminal and the output terminal, in parallel with the series arm.
The fourth SAW resonator adds another pole to the attenuation characteristic of the SAW filter. This pole can be placed to provide additional attenuation at frequencies substantially equal to, or greater than, twice the center frequency of the passband of the SAW filter.